The present invention generally relates to a system and a method for recovering cooling capacity from a factory exhaust gas and more particularly, relates to a system and a method for recovering cooling capacity from a low temperature factory exhaust gas by utilizing its cooling capacity to decrease the temperature of a heat transfer medium used in a cooling tower such that the temperature of the factory exhaust may be increased, the water evaporation in the cooling tower may be reduced and the opacity of the factory exhaust gas may be reduced.
In various semiconductor fabrication processes, the effluent gases from a process chamber must be treated before they can be released into a factory exhaust system and into the atmosphere. It is known that a large number of reactant gases and their reaction products utilized in semiconductor fabrication processes are highly flammable or highly toxic. The spent reactant gases that are discharged out of the process chamber may contain gases that have not been reacted or have been only partially reacted and therefore must be treated before they can be released into the atmosphere.
In a semiconductor fabrication facility, the treatment of the exhaust gases generated from the facility is an important aspect of the total fabrication processes. Various exhaust gases are produced in a semiconductor fabrication facility, these include general exhaust, scrubbed exhaust and solvent exhaust. For discharging the general exhaust and the solvent exhaust, a system typically includes ductworks, exhaust fans, by-passes, and stacks can be used. For handling the scrubbed exhaust, a scrubber must be used for treating the exhaust before they can be released into the atmosphere. A by-pass system can be provided which allows the drawing of outside air when the pressure at the suction side of the blower exceeds a preset value.
In a general exhaust system, heat dissipated by the process equipment is normally removed. The general exhaust therefore does not normally contain acids, caustics or solvents.
In a solvent exhaust system, air containing solvents from the process equipment is removed. The devices utilized in the exhaust system therefore must be explosion-proof for safety reasons. In the scrubbed exhaust system, air containing acids, caustics and other harmful chemicals from the process chamber is removed. Various caustics in the exhaust gases such as ammonia, silane or other toxic gases must be treated by a scrubber before releasing into the atmosphere. A wet scrubber is normally used to remove acids and caustics in a process chamber exhaust by washing the air with a solvent such as water. City water is adequate for such purpose. The waste water from the scrubber is then sent to a neutralization plant in a waste treatment area of the fabrication facility. A dry scrubber can also be used to remove caustics substances from a process chamber exhaust by absorbing the substances into a scrubber material which is typically maintained at an elevated temperature. The scrubber material can then be replaced when it is saturated with the toxic substances.
The various exhaust systems are connected to process machines via ductworks. For instance, when exhausting from a metal etcher, a chemical vapor deposition chamber or a sputter, spent reactant gases and reaction by-products are normally discharged into a scrubbed exhaust system for treatment before the exhaust can be released into the atmosphere. A typical system for treating exhaust gases from a semiconductor process chamber such as an etcher is shown in FIG. 1.
Referring initially to FIG. 1, wherein a semiconductor fabrication system 10 is shown. The fabrication system 10 consists of a process chamber 12, a vent exhaust 14, a main booster pump 16, a dry pump 22, a nitrogen purge gas supply 24 and a wet scrubber 26. Into the process chamber 12, carrier gases and etchant gases (not shown) are first fed into the chamber through various valve openings (not shown). An inert gas such as pure nitrogen is normally used either as a carrier gas for the etchant gases or as a purge gas when venting of the chamber to atmospheric pressure is needed. In a typical metal etching application, etchant gases such as Cl2 and BCl3 are used. In a batch-type metal etcher where a plurality of wafers, i.e., 16 wafers in a column type etcher, are etched in a typical etching process. In order to achieve an effective etching rate for a large number of wafers, a high concentration of etchant gas must be utilized in the process chamber 12. The exhaust gases discharged from the process chamber 12 at the outlet port 18 therefore contains a high concentration of un-spent etchant gases and other etching reaction by-products. The vent exhaust 14 is provided for venting of the pure nitrogen used to purge out the process chamber 12 after an etching reaction. The un-spent etchant gases are discharged out of the process chamber 12 by the main booster pump 16. A dry pump 22 is subsequently used to deliver the un-spent etchant gases into a wet scrubber 26 through an inlet port 20.
The pump exhaust system 30 which includes the main booster pump 16, the dry pump 22 and the dry nitrogen source 24 are controlled by a series of valves (not shown). When the valves between the process chamber 12 and the main booster pump 16 are opened, exhaust gases exit outlet port 18 and pass through the passageway 28 to enter into the main booster pump 16. The main booster pump 16 acts as the front stage pump and the dry pump 22 acts as the back stage pump, which work together to provide a vacuum that is sufficiently high for the process chamber 12 prior to an etching process. The exhaust gases exit the dry pump 22 through passageway 34 and enter the wet scrubber 26 through an inlet port 20. During a normal etching process, chamber 12 is first evacuated by the operation of the main booster pump 16 and the dry pump 22 to a suitable vacuum for conducting the etching process. Etchant gases then enter into the chamber to commence the etching process on the wafers. A suitable chamber pressure is maintained during such etching process.
FIG. 2 is a schematic illustrating a detailed view of the exhaust gas conduit 34 and the wet scrubber 26 shown in FIG. 1. It is seen that exhaust gases 38 delivered from the dry pump 22 enter inlet 42 of the exhaust gas conduit 34. The exhaust gas conduit 34 is normally constructed of stainless steel such that it can be maintained at an elevated temperature of approximately 120xc2x0 C. by heaters 44 to reduce the potential of particulate depositions in the conduit 34. As the exhaust gases 38 enter the wet scrubber 26 through the inlet port 20, the exhaust gases 38 are washed by a cleaning solvent 48 dispensed from a spray head 50. The cleaning solvent 48 is first supplied from a solvent reservoir (not shown) through conduit 52. A commonly used cleaning solvent for a wet scrubber is city water. After being scrubbed by the cleaning solvent 48, the exhaust gases 38 exit the wet scrubber 26 through an exhaust outlet port 46 into a factory exhaust system (not shown). The spent cleaning solvent 38 is collected by the solvent collection device 54 and then transported through conduit 56 into a spent solvent collection tank 58.
It should be noted that, in the application of a wet scrubber for a metal etcher, the spent water collected in the collection tank 58 is maintained at a pH value between about 6 and about 6.3. In other words, the spent city water is allowed to be slightly acidic after it is used to scrub the exhaust gases. The effectiveness of the wet scrubbing operation is maintained by continuously adding fresh city water to the spent water collection tank 58 and recirculating the water through the scrubbing process as long as the pH value of the spent water is between the values described above.
An illustration of the wet scrubber 26 is also shown in FIG. 3 with the additional components of a water pump 64 and an exhaust fan 66. It should be noted that exhaust gas 38 from a process chamber is normally kept at the same temperature as the clean room temperature of 22xc2x12xc2x0 C., while the scrubbed exhaust gas 62 after being washed by city water is approximately 19xc2x0 C., or in the range between about 18xc2x0 C. and about 20xc2x0 C. In this conventional set-up, the scrubbed exhaust gas 62, even though containing a high cooling capacity, is directly released into the atmosphere. The cooling capacity of the scrubbed exhaust gas 62, i.e., at 19xc2x0 C., is therefore completely wasted.
In semiconductor fabrication facilities where clean rooms are widely used, a general exhaust from the clean rooms is normally kept at a low temperature of about 22xc2x12xc2x0 C. The general exhaust must be continuously released into the atmosphere in order to replenish a fresh air environment in the clean rooms. To maintain the clean room temperature at approximately 22xc2x12xc2x0 C., the clean room air is supplied by an air conditioning unit that utilizes cooling towers. A heat exchanger in the cooling tower utilizes chilled water for cooling the clean room air that circulates through the cooling tower. An exhaust air which is blown through the cooling elements of the cooling tower carries the heat transferred from the clean room air to the chilled water. The temperature of the exhaust air from the cooling tower is normally in the range between about 36xc2x0 C. and about 38xc2x0 C. The exhaust air contains a high moisture level, i.e., between about 80% and about 100% relative humidity, more likely between about 90% and about 100% relative humidity. The high humidity content of the cooling tower exhaust air comes from the cooling water that is sprayed on the cooling elements in the tower. The high water content results in wasted consumption of process water in the cooling tower. It is therefore desirable if the moisture content in the cooling tower exhaust gas can be reduced by reducing the temperature of the exhaust air.
In recent years, the minimization of the environment impact by a factory exhaust air has been enforced by various environmental protection agencies. For instance, the opacity of an exhaust air from a manufacturing plant is normally regulated at less than 20%. Since the opacity of the exhaust air is directly related to its temperature and its humidity content, it is desirable not to release exhaust air into the atmosphere which has a temperature lower than 19xc2x0 C., i.e., such as the exhaust air from a wet scrubber.
It is therefore an object of the present invention to provide a system for recovering energy from a factory exhaust gas that does not have the drawbacks or shortcomings of conventional systems.
It is another object of the present invention to provide a system for recovering energy from a factory exhaust gas by utilizing the cooling capacity of a wet scrubber exhaust gas.
It is a further object of the present invention to provide a system for recovering energy from a factory exhaust gas by utilizing the cooling capacity of a wet scrubber exhaust gas and a heat exchanger for cooling a heat transfer medium used in a cooling tower.
It is another further object of the present invention to provide a system for recovering energy from a factory exhaust gas by utilizing a heat exchanger to raise a low temperature wet scrubber exhaust gas to a higher temperature exhaust gas and to reduce the opacity simultaneously.
It is still another object of the present invention to provide a system for recovering energy from a factory exhaust gas by transferring thermal energy from a 38xc2x0 C. cooling tower exhaust gas to a 19xc2x0 C. wet scrubber exhaust gas such that the efficiency of the cooling tower can be improved and the water consumption can be reduced.
It is yet another object of the present invention to provide a system for recovering energy from a factory exhaust gas by recovering cooling capacity from a wet scrubber exhaust gas for converting to use in a water chiller for an air conditioning system.
It is still another further object of the present invention to provide a method for recovering energy from a factory exhaust gas by connecting a heat exchanger between a wet scrubber and a chilled water cooling tower such that the cooling tower water temperature can be reduced by utilizing the cooling capacity in the scrubber exhaust gas.
It is yet another further object of the present invention to provide a method for recovering energy from a factory exhaust gas by transferring heat from a cooling tower water to a wet scrubber exhaust gas such that temperature of the latter is increased by at least 5xc2x0 C. and the opacity of the scrubber exhaust gas is reduced to less than 20%.
In accordance with the present invention, a system and a method for recovering cooling capacity from a factory exhaust gas, and specifically from a low temperature scrubber exhaust gas utilizing a heat exchanger are disclosed.
In a preferred embodiment, a system for recovering energy from a factory exhaust gas can be provided which includes a scrubber that has an inlet for receiving an exhaust gas from a process chamber and an outlet for outputting scrubbed exhaust gas at a first temperature, a chiller in fluid communication with a cooling tower through a first and a second conduit, the chiller further includes a third conduit for outputting a heat transfer medium at a third temperature to a heat exchanger and a fourth conduit for inputting the heat transfer medium at a fourth temperature from the heat exchanger, the fourth temperature is generally lower than the third temperature, and a heat exchanger for receiving the scrubbed exhaust gas at the first temperature and outputting a heated, scrubbed exhaust gas at a second temperature at least 5xc2x0 C. higher than the first temperature by absorbing thermal energy from the heat transfer medium of the chiller.
In the system for recovering energy from a factory exhaust gas, the heated scrubbed exhaust gas outputted from the heat exchanger may be flown into a factory exhaust pipe for releasing into the atmosphere. The first temperature of the outputted scrubbed exhaust gas may be between about 18xc2x0 C. and about 22xc2x0 C. The second temperature of the heated scrubbed exhaust gas may be at least 8xc2x0 C. higher than the first temperature. The second temperature of the heated scrubbed exhaust gas may be at least 28xc2x0 C.
In the system, the fourth temperature may be at least 5xc2x0 C. lower than the third temperature. The chiller may further include an input of chilled water at about 10xc2x0 C. and an output of chilled water at about 5xc2x0 C. The output of chilled water from the chiller may be fed to an air conditioning system. The first conduit transport a heat transfer medium from the cooling tower to the chiller, while the second conduit transport the heat transfer medium from the chiller back to the cooling tower.
In the system, the heat transfer medium transported in the first conduit may be at least 5xc2x0 C. lower in temperature than the heat transfer medium transported in the second conduit. The heat transfer medium transported in the first conduit may be between about 30xc2x0 C. and about 32xc2x0 C., while the heat transfer medium transported in the second conduit may be between about 36xc2x0 C. and about 38xc2x0 C. The heat transfer medium transported in the third conduit may be at least 5xc2x0 C. higher than the heat transfer medium transported in the fourth conduit. The heat transfer medium transported in the third conduit may be between about 36xc2x0 C. and about 40xc2x0 C., while the heat transfer medium transported in the fourth conduit may be between about 26xc2x0 C. and about 30xc2x0 C.
The present invention is further directed to a method for recovering energy from factory exhaust gas which can be carried out by the steps of first providing a scrubber that has a gas inlet and a gas outlet, then flowing an exhaust gas from a process chamber into a gas inlet of the scrubber, flowing a scrubbed exhaust gas at a first temperature not higher than 22xc2x0 C. from the gas outlet of the scrubber into a heat exchanger, providing a chiller in fluid communication with the first conduit and the second conduit, flowing a heat transfer medium at a third temperature to the heat exchanger through a third conduit from the cooling tower, flowing a heat transfer medium at a fourth temperature lower than the third temperature to the cooling tower through a fourth conduit from the heat exchanger, flowing a heated scrubbed gas at a second temperature at least 5xc2x0 C. higher than the first temperature into a factory exhaust pipe for releasing into the atmosphere.
In the method for recovering energy from a factory exhaust gas, the heat transfer medium utilized may be water. The first temperature of the scrubbed exhaust gas may be between about 18xc2x0 C. and about 22xc2x0 C. The energy recovery is accomplished by the heat transfer occurred from the heat transfer medium kept at the third temperature to the scrubbed exhaust gas kept at the first temperature. The second temperature of the heated, scrubbed exhaust gas may be at least 8xc2x0 C. higher than the first temperature, or the second temperature of the heated, scrubbed exhaust gas may be at least 28xc2x0 C. The fourth temperature may be at least 5xc2x0 C. lower than the third temperature. The condenser may further include an input of chilled water at about 10xc2x0 C. and an output of chilled water at about 5xc2x0 C.
In the method, the output of chilled water from the chiller may be fed to an air conditioning system used for cooling clean room air. The first conduit transports a heat transfer medium from the cooling tower to the chiller, while the second conduit transports a heat transfer medium from the chiller to the cooling tower. The heat transfer medium transported in the first conduit may be at least 5xc2x0 C. lower in temperature than the heat transfer medium transported in the second conduit. The same temperature difference may exist in the third conduit and in the fourth conduit. The heat transfer medium transported in the first conduit may be between about 30xc2x0 C. and about 32xc2x0 C., while the heat transfer medium transported in the second conduit may be between about 36xc2x0 C. and about 38xc2x0 C. The same temperature ranges may exist in the third conduit and in the fourth conduit, respectively.